Copper and antimony based material and electrode for the selective conversion of carbon dioxide to carbon monoxide

ABSTRACT

An electrocatalyst material comprising cuprous oxide and antimony, the process for the production thereof and its use in the electrochemical reduction of CO 2  to CO with high selectivity and efficiency are described.

FIELD OF THE INVENTION

The present invention relates to a copper and antimony based material,and an electrode obtained from this material, useful for theelectrochemical reduction of carbon dioxide to carbon monoxide with highefficiency and selectivity.

STATE OF THE ART

Massive emissions of carbon dioxide (CO₂), also known as carbonicanhydride, due to the burning of fossil fuels, have been recognized asresponsible for global climate change. To tackle this problem,strategies such as CO₂ capture and storage are being studied, with theaim of slowing or even stopping the accumulation of CO₂ in theatmosphere. The transformation of captured CO₂ into additionalchemicals, fuels or other products is of paramount importance to achievea sustainable carbon cycle and to store energy in the long term. Amongthe different technologies for CO₂ transformation, electrochemicalconversion is considered particularly interesting since it can useenergy obtained from renewable sources. This technology, although verypromising, is of non-immediate applicability due to the high stabilityof the CO₂ molecule, the slow kinetics and the complex mechanisms of theCO₂ reduction reaction.

CO₂ reduction can occur according to several proton-coupled electrontransfer processes. CO₂ reduction reactions for the production ofcompounds containing a single carbon atom and the electrochemicalevolution of H₂ are reported below as R1-R5, together with theirstandard potentials:

Values of E⁰ are reported under standard conditions (1 atm and 25° C.)with respect to the reversible hydrogen electrode (RHE) in aqueousmedia. Unless otherwise stated, all potentials in this description referto the RHE.

Among the numerous products of CO₂ reduction, formic acid (HCOOH) andcarbon monoxide (CO) are the only economically viable products that havebeen obtained so far with relevant productivity. CO is highly desired inthe industrial sector, since its mixture with hydrogen (H₂), i.e.,synthetic gas or syngas, can be converted into hydrocarbons through theFischer-Tropsch process.

Since, however, the values of the standard potentials of the abovereactions are similar, the result of the process is usually a mixture ofproducts, which is difficult or not easy to use industrially. Inaddition, the parasitic reaction of hydrogen evolution usually occurs inhigher yield than the reduction of CO₂ in aqueous electrolyte.

Therefore, electrode materials are required that can provide high CO₂conversion efficiency and at the same time high selectivity towards aspecific reaction product, in particular towards CO; materials of thiskind are generally known in electrochemistry as electrocatalysts.

According to experimental and theoretical studies, gold (Au), silver(Ag) and palladium (Pd) are considered the best metal electrocatalyststo convert CO₂ into CO; however, these metals cannot be used on anindustrial scale for this purpose due to their high cost and lowavailability.

In addition to the previous materials, the electrocatalytic properties,in CO₂ reduction, of metals such as copper (Cu), zinc (Zn), tin (Sn),indium (In) and bismuth (Bi) have been studied. Cu alone has no goodselectivity for any product; Zn has sufficient, but not optimal,selectivity for CO production; Sn, In and Bi are selective for HCOOHproduction.

In some papers, the properties as electrocatalysts of compositions otherthan single metals are discussed.

Patent Application US 2019/0127866 A1 describes an electrocatalystmaterial for converting CO₂ to ethanol, comprising nanoparticles ofcopper or alloys thereof supported by nanometer-sized tips(“nanospikes”) of carbon doped with nitrogen, boron or phosphorus.Copper alloys indicated as useful by this document are all those of theelement with one or more elements selected from those in the Groups 3-15of the periodic table. Alloys indicated as preferred are those betweencopper and an element selected from Ni, Co, Zn, In, Ag and Sn. Theelectrocatalysts of this document exhibit higher selectivity for CO₂electroreduction than H₂ evolution with high faradic efficiency inethanol production, with a yield in this compound of at least 60% of themixture; other species, such as carbon monoxide, are thus produced withyields not exceeding 40%. In addition to the fact that a mixture ofproducts is produced, the preparation of the doped carbon nanospikesmakes the process not straightforward.

The article “Achieving highly selective electrocatalytic CO₂ reductionby tuning CuO—Sb₂O₃ nanocomposites”, Y. Li et al., ACS Sustainable Chem.Eng. 2020, 8, 12, 4948-4954, describes an electrocatalyst materialcomprising a mixture of carbon in a finely divided form (“carbon black”)and powders of a mixed oxide of copper(II) (CuO) and antimony(III)(Sb₂O₃). The purpose of this study is to identify the best conditionsfor converting CO₂ to CO. The materials in this paper are produced bydissolving soluble Cu(II) and Sb(III) salts in a suspension of carbonblack in ethanol, adding a base (KOH) to the suspension and allowing thesystem to react for 6 hours at a temperature of 80° C. obtained with anoil bath; the precipitate obtained is then washed with water and ethanoland finally dried. The mixture of powders thus obtained is thendistributed on a carbon paper obtaining electrodes. In the section“Results and discussion” of the article, it is confirmed that copperoxide is in the form of CuO (i.e., copper is in oxidation state (II))and that antimony oxide is in the form of Sb₂O₃ (i.e., antimony is inoxidation state (III)), by X-ray diffraction analysis (XRD, FIG. 1 a ofthe article) showing the presence of the characteristic peaks of CuO andSb₂O₃, by X-ray photoelectron spectroscopy (XPS, FIG. 1 b ) and by Ramanspectroscopy (FIG. 1 c ). As shown in the article (see FIG. 3 b ), thebest results are obtained with the molar ratio Cu:Sb 10:1, with whichfaradic yields of approximately 10% for HCOOH, 10% for H₂ and 80% for COare obtained, while the authors report that as the Sb content increases,the CO yield drops rapidly. The results obtained with the best materialof this article are already interesting, but still not optimal both asCO yield and as selectivity towards this compound (a mixture of threeproducts is obtained).

The object of the present invention is to overcome the problems of theprior art, and in particular to provide an electrocatalyst materialwhich allows to obtain in the electrochemical reduction reaction of CO₂a CO yield and a selectivity towards this compound higher than with theelectrocatalysts of the prior art. Another object of the invention is tomake available a cost-effective process for large-scale production ofthis electrocatalyst.

SUMMARY OF THE INVENTION

These objects are achieved with the present invention, which in a firstaspect relates to an electrocatalyst material comprising copper(I) oxide(Cu₂O) containing antimony, wherein the amount of antimony is between 5%to 30% by weight.

This material is used in a finely divided form to produce electrodes forthe electrochemical reduction of CO₂, wherein said material is combinedwith an electroconductive material.

In a second aspect thereof, the invention relates to a process for theproduction of the electrocatalyst material, comprising the followingsteps:

-   a) dissolving a copper(II) salt and an antimony(III) salt in a    solvent selected from ethanol, ethylene glycol, acetylacetone,    diethylamine, ethylenediamine, oleylamine, N,N-dimethylformamide,    mixtures of these solvents with each other, with water or with    aqueous solutions of D-glucose, hydrazine hydrate, amino acids or    sodium carboxymethylcellulose, obtaining a solution;-   b) heating the solution in a microwave oven at a temperature between    180 and 230° C. for a time between 1 and 10 minutes;-   c) separating the precipitate from the solution and its drying.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in detail in the following withreference to the figures, in which:

- FIG. 1 shows photomicrographs obtained by field effect scanningelectron microscope (FESEM) of various materials of the invention andthree comparison materials;

- FIG. 2 shows results of X-ray diffraction (XRD) of powder samples ofmaterials of the invention having different compositions and threecomparison materials;

- FIG. 3 shows spectra obtained by X-ray photoelectron spectroscopy(XPS) for Cu and Sb on a sample of the invention;

- FIG. 4 represents in a schematic form an electrolytic cell used tocarry out the CO₂ reduction tests reported in the Examples section;

- FIG. 5 shows graphs representative of the faradic efficiency in theconversion of CO₂ to CO obtained with a material of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The inventors have found that copper(I) oxide (Cu₂O, cuprous oxide)containing antimony in an amount between 5 and 30% by weight, when usedto produce an electrode, enables the electrochemical reduction of CO₂ toCO to be achieved with higher values of faradic efficiency andselectivity than known materials. The compounds of the invention enablethese results to be obtained by employing copper and antimony, which areinexpensive and widely available components.

A material similar to that of the present invention has been describedin the paper “Optimal synthesis of antimony-doped cuprous oxides forphotoelectrochemical applications”, Dae Yun et al., Thin Solid Films 671(2019) 120-126. However, this paper is directed to the study of theinfluence of Sb concentration on the structural, electrical andphotoelectrochemical properties of cuprous oxide thin films for thepurpose of photoelectrochemical water splitting; besides, this studyreports materials in which the amount of Sb reaches at most up to 1% inmoles, and indicates as a preferred material for the mentioned purposeCu₂O doped with 0.75% molar Sb.

The materials of the invention will generally be referred to in thefollowing by the notation Cu₂O/Sb, regardless of the specificcomposition.

The Cu₂O/Sb materials of the invention have a Sb content between 5 and30% by weight; preferred are the materials having a Sb content between17.2 and 23.9% by weight.

The materials of the invention are obtained and used in powder form. Themorphology of these powders is uniform and homogeneous at least up tothe Sb concentration of 26.4%. FIG. 1 shows images obtained by fieldeffect scanning electron microscope (FESEM) of samples of the inventionwith increasing Sb content (FIG. 1(b) to 1(i)) and, for comparison, ofthree samples produced following the same method as the samples of theinvention but containing only copper (FIG. 1(a)), only antimony (FIG.1(k)), and a sample not of the invention containing an amount ofantimony of 36% (FIG. 1(j)); in particular, the weight percentage amountof Sb in the samples of the invention prepared as described in Example1, determined by chemical analysis, is as follows:

FIG. 1(b): 5.2; FIG. 1(c): 9.4; FIG. 1(d): 13.6; FIG. 1(e): 17.2; FIG.1(f): 20.1; FIG. 1(g): 23.9; FIG. 1(h): 25.2 FIG. 1(i): 26.4.

As can be seen in the images, the materials of the invention with a Sbcontent of up to 26.4% by weight have a similar morphology to oneanother, and comprise powders in the form of essentially sphericalparticles with very narrow size distribution (all particles have a sizeof about 5 µm), composed of tightly packed nanoparticles. Forconcentrations higher than 26.4%, Sb-rich particles and the formation ofan isolated phase consisting of crystalline Sb₂O₃ are observed(octahedral particles in FIG. 1(j), to be compared with the image ofpure antimony oxide in FIG. 1(k)). Energy dispersive X-ray spectroscopy(EDX) analysis indicates that Sb is uniformly distributed in the samplesof the invention.

XRD analysis confirms that the material is essentially copper oxide. InFIG. 2 are shown, from top to bottom, the diffractograms for the samplecontaining only copper (diffractogram indicated with (Cu)), of thesamples of the invention with increasing concentration of antimony(diffractograms from A to H), and of the sample containing 36% by weightof antimony (diffractogram indicated with (NI), which stands for “not ofthe invention”), respectively. As can be seen in the figure, in thesamples of the invention up to a Sb content of 26.4% by weight, onlypeaks attributable to the Cu₂O phase are present (with decreasingintensity as the Sb content increases); in the sample with a Sb contentof 36.0% by weight, peaks attributable to the Sb₂O₃ phase appearinstead, although with low intensity.

The composition is also confirmed by high-resolution (HR) XPSspectroscopy. FIG. 3 shows the typical spectra of the sample containing17.2% by weight of Sb. From the XPS measurement (FIG. 3 a ) it appearsthat antimony is present in the sample in the form of Sb³⁺ ions, ashighlighted by the intense peaks relative to Sb 3d_(5/2) and Sb 3d_(3/2)centred at 530.06 eV and 539.45 eV, respectively. FIG. 3 b shows insteadthe region of the XPS spectrum corresponding to the Cu 2p doublet; sincethe Cu 2p peak is difficult to deconvolve due to the overlap of numerouspeaks, the Auger CuLMM region is also acquired (inset in FIG. 3 b ). Thekinetic energy of the peak is 916.8 eV, which corresponds to Cu⁺. Themodified Auger parameter is about 1848.8 eV, which correlates with anaverage oxidation state of Cu(I). It is therefore evident that copper ispresent in the samples in the form of Cu⁺ ion.

Since the electrocatalyst materials of the invention are poor electricalconductors per se, they are used in combination with conductivematerials for the production of electrodes for CO₂ reduction.Preferably, the conductive material is in turn in the form of powders orother finely divided form. A carbon-based material is generally used forthis purpose, thanks to its low catalytic activity, for example carbonblack, graphite, graphene, carbon nanotubes or mixtures thereof; thepreferred conductive material is carbon black. The electrocatalystmaterial of the invention and the conductive material are used in weightratios between 9:1 and 19:1. For the production of the electrode, themixture between the electrocatalyst material of the invention and theconductive material is distributed on a support, which may in turn beconductive or non-conductive. Examples of preferred supports areconductive carbon paper, conductive carbon cloth and metal mesh.Stabilization of the powder mixture on the support can be achieved withionomers, i.e., ion conductive polymers, which form a containing andconductive film on the powders.

In a second aspect thereof, the invention relates to a process for theproduction of the electrocatalyst material, which consists of steps a)to c) above.

Step a) consists in dissolving a copper(II) salt and an antimony(III)salt in a solvent selected from ethanol, ethylene glycol, acetylacetone,diethylamine, ethylenediamine, oleylamine, N,N-dimethylformamide,mixtures of these solvents with each other, with water or with aqueoussolutions of D-glucose, hydrazine hydrate, amino acids and sodiumcarboxymethylcellulose. The most suitable salts for the purposes of theinvention are acetates, sulfates and nitrates of both metals. Thestarting salts are weighed to obtain the desired weight ratio of Cu:Sb,and thus the desired weight ratio of Cu₂O to Sb; the calculationsnecessary to determine the quantities to be used of the starting salts,given a desired final composition, are of simple executability for theaverage chemist.

The solution thus formed is heated in a microwave oven, within a sealedcontainer of suitable material (e.g., Teflon) at a temperature between180 and 230° C. for a time between 1 and 10 minutes. In addition tocausing the metal salts to react to form the final material, microwaveheating in the presence of the aforementioned solvents results in thereduction of the Cu²⁺ ion of the starting copper salt to Cu⁺ ion presentin the Cu₂O oxide. In the case of ethylene glycol, glycol functions asboth a solvent and a reducing agent, and increasing temperature canincrease its reducing capacity. Normally a temperature between 180° C.and 230° C. is suitable for the formation of Cu⁺ from Cu²⁺ in the givensolution.

Finally, the precipitate formed in the microwave heating is separatedfrom the liquid phase, e.g., by filtration or centrifugation, washedwith ethanol, and dried, e.g., by treatment in an oven at a temperaturebetween 50 and 100° C. under vacuum or in an inert atmosphere.

The process of the invention differs from that of the article by Li etal. cited above in that microwave heating is used instead ofconventional heating, that as said results in the reduction of the Cu²⁺ion of the starting copper salt and the formation of the Cu₂O phase.

The invention will be further described in the experimental sectionbelow.

Materials, Instrumentation and Methods

The following precursors were used in the preparation of the samples:

-   copper(II) acetate, Cu(OAc)₂·xH₂O (Sigma-Aldrich, catalogue No.    66923-66-8 degree of hydration, ~1), 98% purity;-   antimony(III) acetate, Sb(OAc)₃, (Sigma-Aldrich, catalogue No.    6923-52-0), 99.99% purity;-   ethylene glycol (Sigma-Aldrich, catalogue No. 107-21-1), 99.8%    purity;-   Nafion® 117 solution (Sigma-Aldrich, catalogue no. 31175-20-9;    Nafion is a registered trademark of E. I. du Pont de Nemours and    Company), purity: ~ 5% in a mixture of lower aliphatic alcohols and    water.

Chemical composition analyses of the samples were performed byinductively coupled plasma optical emission spectroscopy (ICP-OES, iCAP7600 DUO instrument, Thermo Fisher Scientific); each analysis wasperformed by dissolving 5.0 mg of the sample in 10.0 ml of an aqueoussolution with 10% aqua regia.

Electron microscope images and energy dispersive X-ray spectroscopy(EDX) analyses were obtained with a FESEM Supra 40 (Zeiss) equipped witha detector (Oxford Instruments Si(Li)) for energy dispersive X-rayspectroscopy (EDX) analyses.

The phase composition of each sample was determined by X-ray diffraction(XRD) with a diffractometer (PANalytical X’Pert Pro equipped with anX’Celerator detector) that uses Cu Kα radiation (λ = 1.54178 Å)generated at 40 kV and 30 mA. XRD diffractograms were recorded in the 2025-80° range with a step (20) of 0.017° and a counting time of 0.45seconds.

High-resolution (HR) XPS analyses were performed with a PHI 5000VersaProbe instrument (Physical Electronics) using monochromatic Al Kα(1486.6 eV) radiation.

Analyses of gaseous products derived from CO₂ electroreduction wereperformed in real time with an INFICON Fusion® microgascromatograph(µGC) equipped with two channels with a 10 m Rt-Molsieve 5A column andan 8 m Rt-Q-Bond column, respectively, and thermal conductivitymicrodetectors (micro-TCD).

EXAMPLE 1

This example relates to the synthesis of the materials of the invention.

Seven samples of materials of the invention with different Sb contentswere prepared using copper acetate and antimony acetate as precursors,used in the amounts shown in Table 1. The samples of the invention areindicated as A-H. For comparison, a sample from copper acetate alone(sample referred to as “Cu” in the table), a sample from antimonyacetate alone (sample “Sb”), and a sample of mixed Cu/Sb composition notof the invention (sample “NI”) were also produced in the identicalmanner described below. The last column of the table shows the values ofSb content in each of the samples of the invention, obtained by ICP-OESanalysis (the data for the Cu and Sb samples are not shown becausenaturally in these two cases the analysis for the determination of thepercentage content of Sb was not carried out).

TABLE 1 Sample Amount of precursor (mg) Sb content (% by weight)Cu(OAc)₂·xH₂O Sb(OAc)₃ Cu 900 0 / A 900 164 5.2 B 900 246 9.4 C 900 29513.6 D 900 328 17.2 E 900 410 20.1 F 900 470 23.9 G 900 492 25.2 H 900600 26.4 NI 900 820 36.0 Sb 0 900 /

The indicated amounts of precursors were dissolved in 40 ml of ethyleneglycol and 5 ml of double distilled H₂O (resistivity about 18 MΩ·cm).Each solution was then transferred to a Teflon container (volume 100mL). The Teflon container was sealed, placed in a microwave oven(Milestone, STARTSynth, HPR-1000-10S segment with temperature andpressure control), heated to 220° C. and then maintained at thistemperature by powering the oven with a maximum power of 900 W for atotal irradiation time of 2 minutes. After cooling to room temperature,the suspended product in each container was separated by centrifugationand washed twice with double-distilled H₂O and subsequently once withethanol. Each powder sample was finally dried under vacuum at 60° C.overnight.

In addition to ICP-OES analysis, the samples of the invention wereexamined by scanning electron microscopy and EDX analysis to determinethe morphology (also for Cu and Sb samples) and the antimonydistribution, by X-ray diffraction to determine the crystal structure(also for Cu and Sb samples) and by XPS to determine the oxidation stateof Cu and Sb; the results of the three analyses have been discussedabove with reference to FIGS. 1, 2 and 3 respectively.

EXAMPLE 2

This example relates to the production of electrodes for electrochemicalCO₂ reduction using the materials of the invention (samples A-H) and thethree comparison materials (samples Cu, Sb and NI).

Each electrode was prepared by mixing 10 mg of sample A-H, Cu, Sb or NI,1 mg of carbon black from acetylene, 90 µl of Nafion® 117 solution and320 µl of isopropanol. Each mixture was sonicated for 30 minutes until auniform suspension was obtained. Each suspension was then used to coat acarbon paper covered with a gas permeable layer (GDL; SIGRACET 28BC, SGLTechnologies); the geometric area of each electrode was 1.5 cm². Theobtained electrode was dried at 60° C. overnight to evaporate thesolvents. The electrocatalyst loading on each electrode wasapproximately 3.0 mg cm⁻² The electrodes thus obtained are referred toin the following by the abbreviations E_(x), where the subscript ×corresponds to the sample A-H, Cu, Sb or NIused for its production.

EXAMPLE 3

This example refers to the measurement of the CO₂ reduction efficiencyof the electrodes prepared in the previous Example.

Electrochemical measurements were performed with a cell having theconfiguration schematically shown in FIG. 4 ; the cell as a whole, 10,is shown in the figure enclosed by a discontinuous line. As shown in thefigure, the cell has two compartments separated by an ion exchangemembrane 11 (Nafion® N117 membrane, Sigma-Aldrich), and adopts athree-electrode configuration. Each compartment has a total volume of 10ml and contains 7 ml of electrolyte, and thus 3 ml of headspace. Thereference electrode, 12, is an Ag/AgCl electrode (1 mm, lossless LF-1)that is inserted into the cathode compartment. The counter electrode,13, is a Pt foil (Goodfellow, 99.95%). The working electrode, i.e., theelectrode of the invention, is shown in the figure as element 14. Anaqueous solution of 0.1 M KHCO₃ was used as the electrolyte solution. Inthis configuration, gaseous CO₂ is fed into both half-cells from thelower part of the two compartments, while the mixture of products onwhich the results are evaluated is extracted from the cathodecompartment (on the right in the figure); most of this mixture is sentto the separation and purification stage (performed with methods knownin the field and not described in this text), while a fraction of themixture is sent to the analysis. Chronoamperometric measurements wereperformed using a CHI760D electrochemical workstation (CH Instruments,Inc., USA). Gas phase products were analysed in real time with amicrogascromatograph (µGC). The inlet of the µGC instrument wasconnected to the cathode side of the electrochemical cell through aGENIE filter, to remove humidity from the gas before it entered theanalysis instrument (µGC). During the chronoamperometric measurements,the electrolytes on both sides of the anode and cathode were static,while a constant CO₂ flow rate of 15 ml/min was maintained to saturatethe cathode electrolyte and to bring the gaseous products to the µGC.The tests were performed at different potentials between -0.79 V and-0.99 V. The potential was corrected by compensating for the ohmicpotential drop, 85% of which was from the instrument (iR compensation).

Selectivity is described by the faradic efficiency (FE), which is theratio of the amount of charge (coulomb, C) required to produce a certainamount of a product to the total charge consumed over the reaction time,and is expressed by the following equation:

FE(%) = nNF/Q x 100

where n is the number of electrons transferred in the faradic process(for the reduction of CO₂ to CO and to H₂, n is 2 as shown in thereactions R1 and R5 above), N is the moles of a product generated in aspecific reaction period, F is the faradic constant (96485.33 C/mol),and Q is the total charge in a specific reaction period.

The results of the tests at two potential values are shown in Table 2.

TABLE 2 Electrode Potential -0.79 V Potential -0.99 V FE_(co) (%)FE_(H2) (%) FE_(co) (%) FE_(H2) (%) E_(Cu) 9.5 90 8.5 85 E_(A) 87 14 7326 E_(B) 85 13 84 15 E_(c) 90 8.5 81 18 E_(D) 90 8 92 7 E_(E) 91 8.5 908 E_(F) 90 10.5 89 9.5 E_(G) 89 10 85 14 E_(H) 83.8 16.5 68.5 33 E_(NI)55 43 62 37 E_(Sb) 0 63 0 83

As can be seen from the test results, the E_(Sb) electrode does notproduce CO at either test potential. The Cu electrode has poorselectivity for CO, with FE_(co) values below 10%. The comparison E_(NI)electrode shows poor selectivity values towards CO, probably because itis formed by a mixture containing only a small amount of active materialtogether with a completely inactive material (antimony oxide). Incontrast, the E_(A)-E_(H) electrodes of the invention exhibit highselectivity towards CO, with FEco above 80% for all A-H materials at-0.79 V. Among these materials, in particular, D and E show excellentselectivity values for CO, of at least 90% at both potentials.

EXAMPLE 4

This example relates to the measurement of CO₂ reduction with anelectrode of the invention at various potentials.

The E_(D)electrode, which gave the best results in Example 3, was testedat five different potential values ranging from -0.69 V to -1.09 V. Ineach test, the evolution of CO and H₂ over time was evaluated duringtests lasting between one and two hours.

The results of these tests are shown graphically in FIG. 5 . In detail,FIG. 5(a) to 5(e) report tests performed at the following potentials:5(a) -0.69 V; 5(b) -0.79 V; 5(c) -0.89 V; 5(d) -0.99 V; 5(e) -1.09 V.The tests at -0.79 V and -0.99 V are the same as those whose resultshave already been reported in the previous example. The results of thesetests are provided in summary form in the graph in FIG. 5(f), in whichthe faradic efficiency values for CO and H₂, taken when the reductionprocess has reached steady state, are reported at all evaluatedpotentials.

As can be seen in the graphs (FIG. 5(a)-(e)), in each test there is aninitial settling time between about 10 minutes (test at -0.99 V) and 20minutes; this is attributed to stabilization of the electrode andfilling of the headspace of the electrochemical cell and of tubesbetween the cell and the µGC. Then, the FE values stabilize, indicatingthe stable performance of the electrode. The E_(D)electrode shows verygood performance in the conversion of CO₂ to CO (FE_(co) > 80%) over thewhole range of potentials explored, with values up to 90-92% atpotentials from -0.79 V to -1.09 V. At more negative potentials (< -1.09V), FEco falls below 90%. FE_(H2) values remain low (≤ 9%) from -0.69 Vto -1.09 V. No other gas phase products other than CO and H₂ weredetected. Liquid products (e.g., HCOOH) were not quantified, but can beassumed to be present in very small or negligible amounts, since thetotal faradic efficiency for CO and H₂ measured in all tests is around100%.

COMMENTARY ON THE RESULTS

As demonstrated in the tests described above, the electrocatalystmaterials of the invention catalyze the electrochemical reduction of CO₂with high selectivity toward CO. The materials of the invention thenoffer further advantages.

Firstly, antimony and copper, and the compounds thereof used asprecursors in the process of the invention, are inexpensive materials;moreover, the production of these materials is simple and easilyscalable at an industrial level, also because it does not employ toxicor harmful products; the invention therefore offers a technically viableand competitive alternative to the use of metals such as Au, Ag and Pd.

Since the materials of the invention are in powder form, they can beused in reactors with various configurations as a gas diffusionelectrode (GDE) and different sizes.

1. An electrocatalyst material consisting of copper(I) oxide (Cu₂O)containing antimony, wherein the amount of antimony is between 5% to 30%by weight.
 2. The electrocatalyst material according to claim 1, whereinthe amount of antimony is between 5.2% and 26.4% by weight.
 3. Theelectrocatalyst material according to claim 2, wherein the amount ofantimony is between 17.2% and 23.9% by weight.
 4. An electrodecomprising powder of an electrocatalyst material of claim 1 and aconductive material deposited on a support, in a weight ratio betweenelectrocatalyst material and conductive material between 9:1 and 19:1.5. The electrode according to claim 4, wherein the conductive materialis in the form of powder.
 6. The electrode according to claim 4, whereinthe conductive material is carbon based.
 7. The electrode according toclaim 6, wherein the conductive material is chosen from carbon black,graphite, graphene, carbon nanotubes and mixtures thereof.
 8. Theelectrode according to claim 4, wherein the support is selected fromconductive carbon paper, conductive carbon cloth and metal mesh.
 9. Theelectrode according to claim 4, wherein the powder of theelectrocatalyst material and possibly of the conductive material arestabilized on the support with an ionomer.
 10. A process for theproduction of the electrocatalyst material of claim 1, comprising thefollowing steps: a) dissolving a copper(II) salt and an antimony(III)salt in a solvent selected from ethanol, ethylene glycol, acetylacetone,diethylamine, ethylenediamine, oleylamine, N,N-dimethylformamide,mixtures of these solvents with each other, with water or with aqueoussolutions of D-glucose, hydrazine hydrate, amino acids or sodiumcarboxymethylcellulose, obtaining a solution; b) heating the solution ina microwave oven at a temperature between 180 and 230° C. for a timebetween 1 and 10 minutes; c) separating the precipitate from thesolution and its drying.
 11. The process according to claim 10, whereinthe copper(II) salt is selected from acetate, sulfate and nitrate, andthe antimony(III) salt is selected from acetate, sulfate and nitrate.12. A method for the selective electrochemical reduction of CO₂ to CO,comprising the use of an electrode of claim 4 at a potential between-0.69 V to -1.09 V.